docs: missing note that only changes inside the `src` folders of packages are tracked

[jrson83]

It took me some time to figure out, why I always got the message when running publish:

WARN  No packages have changed since last release, you can run --force to publish all packages.

When I checked the sparkee code, I realized it checks only the packages src folder and package.json for changes:

const { stdout: hasChanges } = await exec(`git diff ${lastTag} -- ${join(folder, 'src')} ${join(folder, 'package.json')}`)

But in my test monorepo I created just an index.js file inside the pk1 root, not in src folder.

Thanks for making this great package. I have tested many publish cli's for monorepos in the last days and I love this one, cause it is so simple.